How do airplanes float on water?

Surely you've all been wondering, here's one answer I ran across (more at the link):All airplanes will eventually sink if it is in water, even pressurized
planes. (more on that later) But there are several areas in the
airplane that have pockets of air that help keep the plane afloat. For
example, in the area between the outside skin of the fuselage and the
interrior there is a space that is usually insulated and has air that
needs to be displaced by the water. In most airplanes built today, the
wing is the fuel tank, and since water is heavier than fuel the fuel in
the wings help offset some of the weight of the plane...not a lot but
some.

There is also air in the cargo hold of larger planes that will help
maintain buoyancy until the air is replaced by water. Anyone who thinks
an airplane is water tight and will float because it is pressurized is
nuts! The airplane is pressurized only while the engines are running
and the air being pumped into the aircraft to pressurize it is almost
escaping the aircraft just as fast as it is being pumped in. There are
control valves in the forward and rear bulkhead that regulate the
pressure inside the plane but all pressure is lost if the engines quit
running. At the altitude that the A-320 that crashed in the Hudson
river was at when it lost it's engines, it probably didn't have much
pressurization anyway since it was only a few thousand feet above sea
level.

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